Antarctica phase VI (2006-2010)

Subject and objectives

The aim of ASPI (Antarctic Subglacial
Processes and Interactions) is
(i) to understand the interactions between the ice sheet and
the subglacial environment and the processes that control the
Antarctic ice sheet, and
(ii) to quantitatively determine the stability of the ice
sheet in a changing climate and the impact of climatic variations
on the coastal ice sheet.

A key factor in such quantification and
impact assessment is the existence of transition zones within
the ice sheet. Such transition zones are examples of specific
boundary layers widely found in glaciology. Basically they
are parts of the ice sheet which overlie basal transition zones
where the flow is anomalous. Typical examples of such transition
zones are the grounding lines, i.e. the interface between the
ice sheet and an ice shelf, between an ice sheet and a subglacial
lake, as well as between an ice shelf and its pinning points.

These
transition zones are probably among the least understood elements
of ice sheets, although they determine to a large extent the
processes and dynamics of lateral expansion and retreat of
ice sheets as well as the stability of marine ice sheets. Apart
from their role in ice dynamics and ice sheet stability, processes
and interactions within basal transition zones also hamper
the interpretation of the paleoclimatic signal as recorded in
deep ice cores. Basal deformation is responsible for disturbing
this signal and understanding the processes at the base of ice
sheets enables such signal recovery (Raynaud et al., 2005). The
subglacial environment opens up new frontiers in Antarctic explorations,
as this dynamic and extreme interface still needs to be explored
in terms of glaciological, geological, geochemical and biological
research efforts.

Together, the present Greenland and Antarctic
ice sheets contain enough water to raise sea level by almost
70 m, so that only a small fractional change in their volume
would already have a significant effect. Changes in ice discharge
generally involve response times of the order of hundreds to
thousands of years. The time-scales are determined by isostasy,
the ratio of ice thickness to yearly mass turnover, processes
affecting ice viscosity, and physical and thermal processes at
the bed (IPCC, 2001). Based on satellite altimetry, Wingham et
al. (1996) observed no surface elevation change in East Antarctica
to within ± 5 mm/yr,
but reported a negative trend in West Antarctica of -53 ± 9
mm/yr, largely located in the Pine Island and Thwaites Glacier
basins. The measurements of Rignot (1998), showing a 1.2 ± 0.3
km/yr retreat of the grounding line of Pine Island Glacier between
1992 and 1996, suggest an ice-dynamic explanation for the observed
thinning.

The West Antarctic Ice Sheet (WAIS) has received
particular attention because it has been the most dynamic part
of the Antarctic ice sheet in the recent geological past, and
because most of it is grounded below sea level – a situation
that, according to models proposed in the 1970s, could lead to
flow instabilities and rapid ice discharge into the ocean when
the surrounding ice shelves would weaken (Thomas, 1973). The
potential of WAIS to collapse in response to future climate change
is still a subject of debate and controversy. According to the
IPCC estimate, major loss of grounded ice, and accelerated sea
level rise, is very unlikely during the 21st century. Nonetheless,
on a longer time-scale, changes in ice dynamics could result
in significantly increased outflow of ice into the ice shelves
and a grounding line retreat. However, processes governing such
grounding line retreat still remain poorly understood.

Therefore,
ASPI seeks to investigate (i) the processes responsible for
grounding line migration in marine ice sheets (present and future
behaviour), (ii) the effect of marine ice formation on the rheology
and ice viscosity of the transition zone, hence the stability
of ice shelves, (iii) the stability of subglacial lakes over
longer time spans, and (iv) basal processes and interactions
in order to unravel the paleoclimatic signal in deep ice cores.